1. Technical Field
The present invention relates to a device and a method for testing a tire, in particular by means of an interferometric measuring method. The object intended to be tested is a tire, but other components could also be tested using the device according to the invention. The measuring method with which the tire is tested is in particular an interferometric measuring method. However, it is also possible to implement other non-destructive measuring methods such as, for example, ultrasound testing or radiographic testing using x-rays. The device has a measuring unit by means of which the tires can be scanned in order to produce a measurement result. The device is also provided with a positioning means to enable the measuring unit to be positioned in a measuring position and to be oriented in a measuring direction.
2. Description of Related Art
Tires or other components that are under load when in use are subjected to material tests for the purposes of quality control and for the reduction of safety risks; these material tests enable faulty areas, known as defects, to be identified. Above all, when used tires are to be remoulded, as a rule a non-destructive type of testing is used which enables relatively rapid series examinations.
Optical measuring methods are frequently to be found in industrial practice such as for example holography or shearography, also commonly known as speckle-pattern-shearing interferometry. Shearography is a relative interferometric measuring method that produces a result image which shows the difference between two chronologically staggered conditions of the test object. In order to generate the resulting digital image generally used nowadays in view of the increasing application of electronic image sensors such as CCD- or CMOS-sensors, it is thus necessary to change the condition of the test object between two measurements by mechanical, thermal or pneumatic forces. To this end, known devices are provided with a pressure chamber which is either evacuated or pressurized so that the test object inside the chamber is deformed as a result of the pressure change and thus transforms from a first reference state to a second measurable state.
In contrast to holography, shearography does not determine the deformation on the surface of a test object but rather measures the gradients of deformation. This is because shearography makes use of what is known as a shearing element which makes use of shear optics such as for example an optical wedge, an optical biprism or a Michelson interferometer, which generates image duplication. As a result of the shearing element, two slightly spatially-shifted images of the test object are produced and superimposed so as to generate an interferogram from the interference obtained. The shearogram characterized by the gradients of the deformation is created by subtraction of the intensities of the interferograms obtained in the reference state and the measured state. The shearogram indicates whether the position of a point in relation to a neighboring point has changed as a result of the deformation of the test object. If it has, then this positional difference leads to a local change in the intensity distribution which gives information about defects. Interferometric measuring methods which are based on this speckle-interferometry are described in DE 42 31 578 A1 and EP 1 014 036 B1.
The devices employed to test a test object by means of an interferometric measuring method generally have at least one measuring head which is provided with a lighting unit and an image-acquisition unit. The lighting unit frequently consists of a coherent-light-emitting laser or laser diode. The image acquisition unit is usually a camera provided with an image sensor, i.e. a light-sensitive semiconductor sensor, for example a CCD- or CMOS-sensor. To achieve meaningful measurement results it is necessary to co-ordinate the camera's optical angle and the section of the test object that is to be tested. Generally, such co-ordination is achieved by positioning the measuring head in a measuring position and orienting it in an observation direction which ensure, on the one hand, that the section of the test object to be tested lies completely within the optical angle of the camera and, on the other hand, that sections to be tested and measured in sequence overlap each other sufficiently to enable a complete and thorough test. The measuring position and the observation direction of the measuring head depend on the dimensions of the test object. Accordingly, a device is known from EP 1 284 409 A1 which enables the test object to be measured optically, for example by means of what are called light sections, so that the measuring head can be positioned and oriented depending on the data acquired in this way.
A tire testing device in which a tire to be tested without a wheel or rim is placed in a pressure chamber in a lying position is disclosed in EP 1 043 578 B1. The tire-testing device is provided with several measuring heads which can be positioned at a prescribed distance from the inner surface of the tire in order to test from the inside the substructure of the tire, i.e. the carcass, the belt frequently incorporated between the carcass and the tread portion as well as the sidewall of the tire. The measuring heads each have a lighting unit and an image acquisition unit and are positioned at an angle to each other so that different sections of the tire can be tested at the same time in order to perform the test comparatively quickly.
The measuring heads are connected to a positioning means which makes it possible to move the measuring heads from a park position outside the tire, which enables the tire for testing to be changed, into a measuring position inside the tire. For this purpose the positioning means is provided with an arm that is movable around the longitudinal axis of the tire and on which the measuring heads are mounted. To enable the measuring heads to be brought into the necessary measuring position and into the desired observation direction, the measuring heads are movable in the radial direction of the tire and they can be swiveled around a pivot axle mounted on the arm.
The known tire-testing device has the disadvantage that, as a result of the arrangement of the measuring heads in the measuring position within the tire, only tires can be tested which have a relatively large internal diameter. Moreover, the arrangement of the measuring heads around a pivot axle, which is absolutely necessary in order to examine fully the internal surface of the tire, has proved to be a disadvantage. The pivotal arrangement of the measuring heads requires a high level of mechanical effort and of control technology, which is associated with cost-intensive production.